This invention relates generally to battery voltage regulator circuits of the type used for regulating the voltage of a battery in an electrical system of a vehicle and more particularly relates to protection circuits for protecting the regulator circuit and the vehicle electrical system from faults, such as misconnection during installation or alternator failures causing an alternator short circuit.
2. Description Of The Related Art
Most vehicles, such as transit buses, automobiles or boats, are powered by internal combustion engines which have an associated electrical system for providing electrical power to the engine and to equipment and accessories on the vehicle. Typically, the electrical system includes an alternator mechanically driven by the internal combustion engine for providing a charging current to the battery. A battery regulator senses battery voltage and switches the connection of the alternator field winding to the battery to control battery-charging current in order to assure that full battery charge is maintained without overcharging the battery and damaging it.
In the past, regulators were principally electromechanical devices which included relay-type structures and wirewound resistors. As electronic semiconductor devices have become both more sophisticated, less expensive and capable of operating at higher currents, electronic regulators have replaced the older electromechanical devices. Although electronic regulators offer improved performance, they are at greater risk of damage from a misconnection which can occur during installation of the regulator or from an electrical system failure because semiconductor devices are more easily damaged by excessive currents and reverse polarity misconnection. Since regulators are ordinarily three or four terminal devices, each terminal of which is supposed to be connected to its mating terminal in the electrical system, there are a variety of combinations of misconnections which are possible.
It is therefore a principal object and feature of the present invention to provide protective circuitry for protecting the regulator against misconnection and against alternator defects which occur during operation.
More particularly, it is an object and feature of the invention to protect the components of the regulator, so that no matter how the regulator circuit is misconnected in the vehicle electrical system, the regulator will not be damaged and therefore can be used when it is subsequently properly connected.
FIG. 1 illustrates the principles of prior art regulator circuitry 18. In a conventional electrical system, an alternator armature 10 is connected through a rectifier diode 12 to a battery 14 for charging the battery. The field winding 16 of the alternator is connected to a field terminal FLD and a ground terminal GND. The regulator circuit 18 receives its power from the battery 14 through an input terminal BAT connected directly to the battery, or, in some circuits, the input terminal is instead connected to an ignition terminal, often labeled IGN on the vehicle starting switch. An ignition switch terminal provides battery voltage to the regulator circuit 18, except when the ignition is turned off, as is well known in the art.
The prior art regulator circuit has a controlled power switch 22 in series with the field winding 16 and is connected either directly to the battery 14 or indirectly to the battery 14 through an ignition switch. The controlled power switch 22 is controlled by a voltage regulating circuit 24, which has an input 26 connected across the battery 14 for detecting the voltage of the battery 14. The voltage regulating circuit 24 closes the controlled power switch 22 when the voltage of battery 14 is below a selected voltage to apply battery voltage to the field 16 and opens the controlled power switch 22 when the battery voltage is above a selected voltage level.
The terms "switch", "controlled switch" and "controlled power switch" are used in the broad electronic sense as referring to a variety of switching devices which have a control input for switching them between their states. These include the well-known examples of relays and transistors which are commonly utilized as switches. The control inputs are typically the coil of a relay, the gate of a MOSFET transistor or the base or other terminal of a bipolar transistor, all of which are used in well-known transistor circuit configurations for providing a switching function. These circuits usually include supporting circuit elements, such as bias and current limiting resistors and protection diodes. Typically electronic switches have inputs connected to switch control circuits or drive circuits, of which there are many types. Typically, switch control circuits receive an input signal, such as a voltage or current, and have a reference voltage or current for comparison to the input voltage or current. They turn the switch on or off as the voltage or current magnitude rises above or falls below a set point reference. Switch drive circuits may include or be only a voltage, current or impedance transforming circuit or biasing circuit for changing magnitudes to values acceptable to the inputs of the device being driven.
Because the field winding 16 of an alternator is an inductor, when the controlled power switch 22 is opened the voltage across the field winding 16 will reverse polarity and the magnetic field will begin to collapse to maintain current flow through the field winding 16. A flyback diode 28 is provided as a path for that current flow as the magnetic field decays when the controlled power switch 22 is opened.
In the operation of this prior art circuit, when the battery is fully charged, the controlled power switch 22 may be switched on and off, typically at a 100-200 Hz rate, alternately applying the voltage of battery 14 to the field 16 at that rate and maintaining an essentially constant current through the field 16. If, however, the battery is discharged, the controlled power switch will be held on continuously and if the battery is overcharged, it will be held off continuously.
A few prior art regulators have an overvoltage protection circuit 30. Such a circuit has a voltage responsive switch which senses battery voltage and is connected in series in the circuit branch between the BAT terminal and the FLD terminal. This switch opens when the battery exhibits an overvoltage. However, the prior art overvoltage protection circuit 30 is not resettable. It is interposed in the circuit branch in order to protect the electrical system of the vehicle. Such prior art regulators must be sent back for servicing in order to be repaired or replaced before subsequent use. Consequently, such a circuit is not practical for protection against misconnection since misconnection effectively disables such a prior art regulator.
It is therefore another object and feature of the invention to provide protection circuitry which protects both the vehicle electrical system and the regulator circuit itself and does so in a manner which is entirely resettable so that once the misconnection is corrected or the defective electrical system component is repaired, the same voltage regulator may be installed and reused without requiring prior servicing.